A field emission device (FED) is widely employed as a field emission display, as an electron source of, for example, a scanning electron microscope (SEM) or transmission electron microscope (TEM), as an X-ray generator, as a gas ionizer, etc.
Typically, the FED applies an external electric field to a surface of an electron emitter so that electrons on the surface are emitted outward using quantum-mechanical tunneling. Various electron-emitting cathodes formed of a carbon-based material, metal or alloy may be used as the electron emitter for emitting electrons.
Meanwhile, electrons emitted from the electron emitter are changed into a form of electron beams and may be used for the field emission display, the SEM, the TEM, etc., as mentioned above. Moreover, an electric field or a magnetic field is separately applied to the emitted electrons to change the emitted electrons into the form of controlled electron beams. An X-ray tube including a field emitter having a carbon nanotube, a gate electrode, an anode, a solenoid lens, and an X-ray target is disclosed in S. H. Heo et al, “Applied Phys. Lett. 90, 183109 (2007).” The carbon nanotube formed on a tungsten tip emits electrons in response to an applied voltage. The gate electrode or the anode generates an electric field, and the solenoid lens generates a magnetic field. The electric field and the magnetic field modify the emitted electrons to be focused electron beams. Accordingly, the focused electron beams impact with the X-ray target to produce an X-ray.
One drawback is that a separate device is required for generating the electric or magnetic field to control the electron beams, which makes the whole structure complicated and costly to manufacture.